If you were to treat a fish in a bucket, you'd need a completely new one.And an airstone obviously.I don't know an awful lot about seperate treatment - If one of mine came down ill, I'd put him/her in my spare tank with new water (and dechlor with some prime) and do waterchanges everyday, with meds as it says.

Cotton-wool tufts, the slime bacterium Chondrococcus causes this. It is unrelated to body fungus which is the bacterium Saprolegnia. Treatment for both should be carried out in the tank where it originated as well as hospital tanks if individual fish are isolated. There are quite a few propritary brands of fungus control all of which state they are the one you need. What I suggest is that you buy one that contains Phenoxethol. Barring that the only other treatment I am aware of are antibiotics. For the latter you would need to seek a vets advice.

Buy a bottle of eSHa 2000 from your LFS, it's not the cheapest, but IMO it is the best all round treatment for fungus type ailments (and a lot of others too like slime, ulcers etc..). While you are at it best to buy two bottles, one for now and one for any later emergencies.

Treat insitu is best, as others have said.

Mouth fungus often starts where there is damage to the mouth... things to look for: